What are the negative health effects of masturbation, if any?

There really aren't: If there were, both men and women would be a lot less healthy overall. Anyone telling you it will make you short, fat, stupid, hairy, smooth, unspiritual, an unfit huisband / father, sterile, nearsighted, demented, or whatever else is lying to you for their own dark purposes. Think about that one special person in your future rather than internet trash. Enjoy being human.